How to change an unknown diameter of a hole to a specific size?
Another way to achieve this is using the LoopTools addon's 'Circle tool'.

If the addon is not active on your system, go to the User Settings (CtrlAltU) > Addons > Mesh > LoopTools and activate it using the small checkbox on the right:



In Edit Mode select the loop or loops you want to resize, then either from the Tool Shelf > LoopTools or the Specials Menu (W > LoopTools) choose the 'Circle tool':



Toggle the "Radius" option on, then type in the radius you want (diameter / 2) for the hole: